This invention relates to a method of refining hydroxide of niobium or tantalum containing transition metals as impurities by treatment of a solution of the crude hydroxide with a specially selected reagent.
Recently niobium and tantalum and their carbides have been used in the manufacture of optical devices and electronic devices, and usually oxides of neobium and tantalum are used as the raw materials of the metals and carbides. Therefore, there is an increasing demand for niobium oxide and tantalum oxide of very high purity.
Usually oxides of niobium and tantalum are prepared from hydroxides of niobium and tantalum, respectively. That is, oxide of niobium or tantalum is prepared by the steps of decomposing an ore containing the metal by a known process using suitable acids such as sulfuric acid and hydrofluoric acid and performing solvent extraction to obtain a solution of a niobium or tantalum fluoride complex, adding a suitable alkali such as ammonia to the solution to precipitate hydroxide of the metal, and drying and firing the precipitated hydroxide.
However, hydroxide of niobium or tantalum obtained by the above process contains considerable amounts of impurities including various transition metals, and most of the impurity metals remain in the metal oxide obtained from the hydroxide. As a raw material for optical or electronic devices, the metal oxide is required to be very low in the contents of transition metals. To meet the requirement it is necessary to take a special measure for enhancement of the purity of the hydroxide prepared by the above described process. For example, it is necessary to remove metal impurities from the metal fluoride complex solution before precipitating the metal hydroxide by performing solvent extraction or ion exchange treatment (e.g., JP No. 38-13607, JP No. 38-18061, JP No. 56-114831). However, the inclusion of such a purification operation renders the hydroxide preparing process complicated and raises the cost of production.